Within the field of pressure-sensitive adhesives (PSAs), ongoing technological developments in the coating process mean that there is a continual need for new developments. In the industry, hotmelt processes with solvent-free coating technology are of increasing importance for the preparation of PSAs, since the environmental regulations are becoming ever greater and the prices of solvents continue to rise. Consequently, solvents are to be eliminated as far as possible from the manufacturing operation for PSA tapes. As a result of the associated introduction of the hotmelt technology, the requirements imposed on the adhesives are becoming every more stringent. Acrylate PSAs in particular are the subject of very intensive investigations aimed at improvements. For high-end industrial application preference is given to polyacrylates, on account of their transparency and weathering stability. As well as these advantages, however, these acrylate PSAs must also meet exacting requirements in respect of shear strength and bond strength. This profile of requirements is matched by polyacrylates of high molecular weight and high polarity with subsequent efficient crosslinking. The drawback with these high-shear-strength, polar PSAs, however, is that they are unsuited to the operation of hotmelt extrusion, since, as a result of the high flow viscosity, high application temperatures are necessary and, moreover, the molecular weight of the polymer is reduced by shearing within the extruder. This damage significantly lowers the level of adhesive performance. The bond strength and the tack are generally low, since the glass transition temperature is relatively high because of the polar fractions in the adhesives. The shear strengths in particular of hotmelt-coated acrylate PSAs drop significantly in comparison to the original solvent-coated PSA. At the present time, therefore, various concepts are being investigated with the aim of reducing flow viscosity and hence of facilitating the extrusion coating of these PSAs.
One very important concept is the targeted adjustment of the molecular weight distribution for the purpose of improved processing. Bimodal molecular weight distributions assist easier processing, since low molecular weight fractions lower the flow viscosity while high molecular weight fractions raise the shear strength. Bimodal molecular weight distributions are generally produced by means of targeted blending. In U.S. Pat. No. 5,548,014 polyolefin blends are prepared by this method and processed in a hotmelt process. The general drawback of this method is that in two process steps the polymers, first, must be prepared, with their different average molecular weight, and subsequently must be mixed. A process for such blending of polyolefins is described in U.S. Pat. Nos. 5,863,665 and 5,773,155. It is also necessary here to introduce a relatively large amount of energy in order to mix the systems with one another, since the miscibility of polymers with one another is relatively poor, because of their long polymer chains.
Furthermore, bimodal molecular weight distributions have been disclosed for starch degradation in polyacrylate dispersions (U.S. Pat. No. 6,084,018, U.S. Pat. No. 6,080,813, U.S. Pat. No. 5,705,563). There is no connection here, though, neither with the elastomer nor with an operation of hotmelt extrusion.
U.S. Pat. Nos. 4,619,979 and 4,843,134 describe a preparation process for the solvent-free polymerization of acrylates. Here, in a specific polymerization reactor, highly branched polymers were prepared. Drawbacks are the high gel fraction formed during the polymerization, which although allowing a bimodal molecular weight distribution makes it impossible to coat such a material, and the low conversion of the polymerization, resulting in the need to remove acrylate monomers from the system again, an operation which is relatively costly and inconvenient. Since acrylate PSAs are generally composed of two or more comonomers, and these comonomers possess different boiling temperatures and vapor pressures, this is a very costly and inconvenient process.
It is an object of the invention to avoid the drawbacks which exist in the prior art and to provide a broad, bimodally distributed polyacrylate having good hotmelt processing properties while retaining good adhesive properties. The polyacrylate ought preferably to have a low residual monomer content.
This object is achieved by a polymerization process according to the main claim, and also by a polyacrylate and its use according to the coindependent claims. Advantageous developments of the invention are characterized in the dependent claims.